There are car navigation apparatuses in the related art that are equipped with a function of displaying a roadmap of an area around the vehicle position, a function of calculating a recommended route from a start point to a destination, a function of providing route guidance based upon the calculated recommended route and the like. Since such a car navigation apparatus needs to detect the current position of the vehicle (the vehicle position), it includes a current position detection device constituted of, for instance, an azimuth sensor that detects the advancing azimuth of the vehicle, a vehicle speed sensor that detects the vehicle speed, a GPS sensor that detects a GPS signal from a GPS (global positioning system) satellite and the like.
Since there is a specific margin of error in the accuracy of the current position detection device, the car navigation apparatus executes correction by adopting the so-called map matching method of the known art if the vehicle position obtained by the current position detection device is not on a road in the roadmap data, so as to set the vehicle position on a road. Through this correction, the vehicle position is displayed on the roadmap with a high degree of accuracy at all times.
We sometimes find ourselves at a roundabout while driving. A roundabout, which is also called a traffic circle or a rotary, is a circular crossroad at the center of a plurality of radially extending roads. De Gaul Square in Paris is a well-known example of a roundabout. The Arc De Triumph is located at the center of De Gaul Square, and the Arc De Triumph is circled by a circle road on which the traffic can advance along a specific direction. Twelve streets such as the Champs Elysee extends radially from De Gaul Square, i.e., from the circle road. A vehicle which has entered De Gaul Square from one of the twelve streets drives around the square on the circle road and drives out into another radially extending street as desired.
However, when map matching processing is executed while the vehicle is traveling through such a roundabout, the position of the vehicle entering a radial road from the roundabout may be erroneously indicated on an adjacent radial road. In addition, when the map matching processing is executed when the vehicle is entering a street from a small roundabout, its position may be erroneously indicated on the radial road from which the vehicle entered the roundabout.